There has been known a tandem-type image forming apparatus such as color printer in which photosensitive drums corresponding to respective colors of yellow, magenta, cyan and black are arranged side by side in a conveyance direction of a sheet (refer to JP-A-2010-32722). In this image forming apparatus, transfer rollers are provided in correspondence to the respective photosensitive drums and a transfer bias is applied to the transfer rollers, so that toner on the photosensitive drums is transferred to the sheet when the sheet is conveyed between the photosensitive drums and the transfer rollers.
In the tandem-type image forming apparatus, when toner of a first color transferred to the sheet is aggregated, a potential of the corresponding part could be increased. When it is intended to overlap and transfer toner of another color onto the toner of the first color having the increased potential, the toner of the other color would be scattered by the toner of the first color. This problem becomes conspicuous when a charge amount of the toner of the first color is increased (hereinafter, which is referred to as ‘charge up’) as the toner passes between a downstream photosensitive drum and a transfer roller, and the toner from the photosensitive drum arranged at the most downstream side is transferred with being overlapped onto the toner of the first color.